Gas Chromatography (GC) (IUPAC Compendium of Chemical Terminology):
Lecture 3. Gas chromathography. The Retention Index. Each chromatographic setup will vary to some degree. Retention times for a known set of species can be hard to reproduce even from instrument to another. Retention indexing helps to standardize the results. For alkanes C n H 2n+2 : n is proportional to log t' R By agreement, Kovats retention index for linear alkanes equals 100 times the number of carbon atoms. For the compound eluted between two linear alkanes with number of atoms n and N=n+1, is: I =100 [ nN −n log t ' Runknown−log t ' R n log t ' R N −log t ' R n ] Example: t R (methane) = 0.5 min t R (octane) = 14.3 min t R (unknown) = 15.7 min t R (nonane) = 18.5 min Find the retention index for unknown. SOLUTION: t' R (octane) = 14.3 – 0.5 = 13.8 min t' R (unknown) = 15.7 – 0.5 = 15.2 min t' R (nonane) = 18.5 – 0.5 = 18.2 min [ log 15.2−log 13.8 ] I unknown =100 89−8 log 18.0−log 13.8 =836 Kovats retention indexes must be compared on the same or very similar phases. For phases with different polarity the order of elution and, therefore, the retentions indexes are very different! 18
Lecture 3. Gas chromathography. Temperature programming. By definition, programmed-temperature chromatography (temperature programming) A procedure in which the temperature of the column is changed systematically during a part or the whole of the separation. As stated before, the retention time of homologues increases exponentially with the number of carbon. With longer retention time, the peaks are broad and wide, making detection difficult or even impossible. C 10 C 9 C 12 C 8 C 11 C 13 C Isotermal 150 ºC 14 C 15 0 10 20 30 40 50 60 70 80 90 100 time, min Raising the column temperature: ● decrease retention time; ● sharpens peak. Factors to take into account for temperature programming: ● Stability of stationary phase ● Stability of solutes ● Changes in flow rates ● Changes in solute volatility ● Changes in solute solubility Steps to create a temperature program: 1. Determine initial temperature and time according to best possible separation of fast peaks. 2. Determine final temperature according to best possible separation of last peaks. 3. Find experimentally the optimal temperature gradient to account the middle peaks. C 10 C 11 C 14 C 15 C 12 C 13 C 8 C 9 Programmed temperature 50 – 250 ºC at 8º/min C 16 C 17C18 C 19 C 20 C 21 C 6 C 7 0 4 8 12 20 36 16 24 28 32 time, min 19
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Lecture 3. <strong>Gas</strong> chromathography.<br />
Temperature programming.<br />
By definition, programmed-temperature chromatography (temperature programming)<br />
A procedure in which the temperature <strong>of</strong> the column is changed systematically during a part or<br />
the whole <strong>of</strong> the separation.<br />
As stated before, the retention time <strong>of</strong> homologues increases exponentially with the number <strong>of</strong><br />
carbon. With longer retention time, the peaks are broad and wide, making detection difficult or<br />
even impossible.<br />
C 10<br />
C 9<br />
C 12<br />
C 8<br />
C 11<br />
C 13<br />
C Isotermal 150 ºC<br />
14<br />
C 15<br />
0 10 20 30 40 50 60 70 80 90 100<br />
time, min<br />
Raising the column temperature:<br />
● decrease retention time;<br />
● sharpens peak.<br />
Factors to take into account for temperature programming:<br />
● Stability <strong>of</strong> stationary phase<br />
● Stability <strong>of</strong> solutes<br />
● Changes in flow rates<br />
● Changes in solute volatility<br />
● Changes in solute solubility<br />
Steps to create a temperature program:<br />
1. Determine initial temperature and time according to best possible separation <strong>of</strong> fast peaks.<br />
2. Determine final temperature according to best possible separation <strong>of</strong> last peaks.<br />
3. Find experimentally the optimal temperature gradient to account the middle peaks.<br />
C 10<br />
C 11<br />
C 14<br />
C 15<br />
C 12<br />
C 13<br />
C 8<br />
C 9<br />
Programmed temperature<br />
50 – 250 ºC at 8º/min<br />
C 16<br />
C 17C18<br />
C 19 C 20 C 21<br />
C 6 C 7<br />
0 4 8 12 20 36<br />
16 24 28 32<br />
time, min<br />
19
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